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  • J.U.C并发框架源码阅读(二)AbstractQueuedSynchronizer

    基于版本jdk1.7.0_80

    java.util.concurrent.locks.AbstractQueuedSynchronizer

    代码如下

    /*
     * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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    /*
     *
     *
     *
     *
     *
     * Written by Doug Lea with assistance from members of JCP JSR-166
     * Expert Group and released to the public domain, as explained at
     * http://creativecommons.org/publicdomain/zero/1.0/
     */
    
    package java.util.concurrent.locks;
    import java.util.*;
    import java.util.concurrent.*;
    import java.util.concurrent.atomic.*;
    import sun.misc.Unsafe;
    
    /**
     * Provides a framework for implementing blocking locks and related
     * synchronizers (semaphores, events, etc) that rely on
     * first-in-first-out (FIFO) wait queues.  This class is designed to
     * be a useful basis for most kinds of synchronizers that rely on a
     * single atomic <tt>int</tt> value to represent state. Subclasses
     * must define the protected methods that change this state, and which
     * define what that state means in terms of this object being acquired
     * or released.  Given these, the other methods in this class carry
     * out all queuing and blocking mechanics. Subclasses can maintain
     * other state fields, but only the atomically updated <tt>int</tt>
     * value manipulated using methods {@link #getState}, {@link
     * #setState} and {@link #compareAndSetState} is tracked with respect
     * to synchronization.
     *
     * <p>Subclasses should be defined as non-public internal helper
     * classes that are used to implement the synchronization properties
     * of their enclosing class.  Class
     * <tt>AbstractQueuedSynchronizer</tt> does not implement any
     * synchronization interface.  Instead it defines methods such as
     * {@link #acquireInterruptibly} that can be invoked as
     * appropriate by concrete locks and related synchronizers to
     * implement their public methods.
     *
     * <p>This class supports either or both a default <em>exclusive</em>
     * mode and a <em>shared</em> mode. When acquired in exclusive mode,
     * attempted acquires by other threads cannot succeed. Shared mode
     * acquires by multiple threads may (but need not) succeed. This class
     * does not &quot;understand&quot; these differences except in the
     * mechanical sense that when a shared mode acquire succeeds, the next
     * waiting thread (if one exists) must also determine whether it can
     * acquire as well. Threads waiting in the different modes share the
     * same FIFO queue. Usually, implementation subclasses support only
     * one of these modes, but both can come into play for example in a
     * {@link ReadWriteLock}. Subclasses that support only exclusive or
     * only shared modes need not define the methods supporting the unused mode.
     *
     * <p>This class defines a nested {@link ConditionObject} class that
     * can be used as a {@link Condition} implementation by subclasses
     * supporting exclusive mode for which method {@link
     * #isHeldExclusively} reports whether synchronization is exclusively
     * held with respect to the current thread, method {@link #release}
     * invoked with the current {@link #getState} value fully releases
     * this object, and {@link #acquire}, given this saved state value,
     * eventually restores this object to its previous acquired state.  No
     * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a
     * condition, so if this constraint cannot be met, do not use it.  The
     * behavior of {@link ConditionObject} depends of course on the
     * semantics of its synchronizer implementation.
     *
     * <p>This class provides inspection, instrumentation, and monitoring
     * methods for the internal queue, as well as similar methods for
     * condition objects. These can be exported as desired into classes
     * using an <tt>AbstractQueuedSynchronizer</tt> for their
     * synchronization mechanics.
     *
     * <p>Serialization of this class stores only the underlying atomic
     * integer maintaining state, so deserialized objects have empty
     * thread queues. Typical subclasses requiring serializability will
     * define a <tt>readObject</tt> method that restores this to a known
     * initial state upon deserialization.
     *
     * <h3>Usage</h3>
     *
     * <p>To use this class as the basis of a synchronizer, redefine the
     * following methods, as applicable, by inspecting and/or modifying
     * the synchronization state using {@link #getState}, {@link
     * #setState} and/or {@link #compareAndSetState}:
     *
     * <ul>
     * <li> {@link #tryAcquire}
     * <li> {@link #tryRelease}
     * <li> {@link #tryAcquireShared}
     * <li> {@link #tryReleaseShared}
     * <li> {@link #isHeldExclusively}
     *</ul>
     *
     * Each of these methods by default throws {@link
     * UnsupportedOperationException}.  Implementations of these methods
     * must be internally thread-safe, and should in general be short and
     * not block. Defining these methods is the <em>only</em> supported
     * means of using this class. All other methods are declared
     * <tt>final</tt> because they cannot be independently varied.
     *
     * <p>You may also find the inherited methods from {@link
     * AbstractOwnableSynchronizer} useful to keep track of the thread
     * owning an exclusive synchronizer.  You are encouraged to use them
     * -- this enables monitoring and diagnostic tools to assist users in
     * determining which threads hold locks.
     *
     * <p>Even though this class is based on an internal FIFO queue, it
     * does not automatically enforce FIFO acquisition policies.  The core
     * of exclusive synchronization takes the form:
     *
     * <pre>
     * Acquire:
     *     while (!tryAcquire(arg)) {
     *        <em>enqueue thread if it is not already queued</em>;
     *        <em>possibly block current thread</em>;
     *     }
     *
     * Release:
     *     if (tryRelease(arg))
     *        <em>unblock the first queued thread</em>;
     * </pre>
     *
     * (Shared mode is similar but may involve cascading signals.)
     *
     * <p><a name="barging">Because checks in acquire are invoked before
     * enqueuing, a newly acquiring thread may <em>barge</em> ahead of
     * others that are blocked and queued.  However, you can, if desired,
     * define <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to
     * disable barging by internally invoking one or more of the inspection
     * methods, thereby providing a <em>fair</em> FIFO acquisition order.
     * In particular, most fair synchronizers can define <tt>tryAcquire</tt>
     * to return <tt>false</tt> if {@link #hasQueuedPredecessors} (a method
     * specifically designed to be used by fair synchronizers) returns
     * <tt>true</tt>.  Other variations are possible.
     *
     * <p>Throughput and scalability are generally highest for the
     * default barging (also known as <em>greedy</em>,
     * <em>renouncement</em>, and <em>convoy-avoidance</em>) strategy.
     * While this is not guaranteed to be fair or starvation-free, earlier
     * queued threads are allowed to recontend before later queued
     * threads, and each recontention has an unbiased chance to succeed
     * against incoming threads.  Also, while acquires do not
     * &quot;spin&quot; in the usual sense, they may perform multiple
     * invocations of <tt>tryAcquire</tt> interspersed with other
     * computations before blocking.  This gives most of the benefits of
     * spins when exclusive synchronization is only briefly held, without
     * most of the liabilities when it isn't. If so desired, you can
     * augment this by preceding calls to acquire methods with
     * "fast-path" checks, possibly prechecking {@link #hasContended}
     * and/or {@link #hasQueuedThreads} to only do so if the synchronizer
     * is likely not to be contended.
     *
     * <p>This class provides an efficient and scalable basis for
     * synchronization in part by specializing its range of use to
     * synchronizers that can rely on <tt>int</tt> state, acquire, and
     * release parameters, and an internal FIFO wait queue. When this does
     * not suffice, you can build synchronizers from a lower level using
     * {@link java.util.concurrent.atomic atomic} classes, your own custom
     * {@link java.util.Queue} classes, and {@link LockSupport} blocking
     * support.
     *
     * <h3>Usage Examples</h3>
     *
     * <p>Here is a non-reentrant mutual exclusion lock class that uses
     * the value zero to represent the unlocked state, and one to
     * represent the locked state. While a non-reentrant lock
     * does not strictly require recording of the current owner
     * thread, this class does so anyway to make usage easier to monitor.
     * It also supports conditions and exposes
     * one of the instrumentation methods:
     *
     * <pre>
     * class Mutex implements Lock, java.io.Serializable {
     *
     *   // Our internal helper class
     *   private static class Sync extends AbstractQueuedSynchronizer {
     *     // Report whether in locked state
     *     protected boolean isHeldExclusively() {
     *       return getState() == 1;
     *     }
     *
     *     // Acquire the lock if state is zero
     *     public boolean tryAcquire(int acquires) {
     *       assert acquires == 1; // Otherwise unused
     *       if (compareAndSetState(0, 1)) {
     *         setExclusiveOwnerThread(Thread.currentThread());
     *         return true;
     *       }
     *       return false;
     *     }
     *
     *     // Release the lock by setting state to zero
     *     protected boolean tryRelease(int releases) {
     *       assert releases == 1; // Otherwise unused
     *       if (getState() == 0) throw new IllegalMonitorStateException();
     *       setExclusiveOwnerThread(null);
     *       setState(0);
     *       return true;
     *     }
     *
     *     // Provide a Condition
     *     Condition newCondition() { return new ConditionObject(); }
     *
     *     // Deserialize properly
     *     private void readObject(ObjectInputStream s)
     *         throws IOException, ClassNotFoundException {
     *       s.defaultReadObject();
     *       setState(0); // reset to unlocked state
     *     }
     *   }
     *
     *   // The sync object does all the hard work. We just forward to it.
     *   private final Sync sync = new Sync();
     *
     *   public void lock()                { sync.acquire(1); }
     *   public boolean tryLock()          { return sync.tryAcquire(1); }
     *   public void unlock()              { sync.release(1); }
     *   public Condition newCondition()   { return sync.newCondition(); }
     *   public boolean isLocked()         { return sync.isHeldExclusively(); }
     *   public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
     *   public void lockInterruptibly() throws InterruptedException {
     *     sync.acquireInterruptibly(1);
     *   }
     *   public boolean tryLock(long timeout, TimeUnit unit)
     *       throws InterruptedException {
     *     return sync.tryAcquireNanos(1, unit.toNanos(timeout));
     *   }
     * }
     * </pre>
     *
     * <p>Here is a latch class that is like a {@link CountDownLatch}
     * except that it only requires a single <tt>signal</tt> to
     * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt>
     * acquire and release methods.
     *
     * <pre>
     * class BooleanLatch {
     *
     *   private static class Sync extends AbstractQueuedSynchronizer {
     *     boolean isSignalled() { return getState() != 0; }
     *
     *     protected int tryAcquireShared(int ignore) {
     *       return isSignalled() ? 1 : -1;
     *     }
     *
     *     protected boolean tryReleaseShared(int ignore) {
     *       setState(1);
     *       return true;
     *     }
     *   }
     *
     *   private final Sync sync = new Sync();
     *   public boolean isSignalled() { return sync.isSignalled(); }
     *   public void signal()         { sync.releaseShared(1); }
     *   public void await() throws InterruptedException {
     *     sync.acquireSharedInterruptibly(1);
     *   }
     * }
     * </pre>
     *
     * @since 1.5
     * @author Doug Lea
     */
    public abstract class AbstractQueuedSynchronizer
        extends AbstractOwnableSynchronizer
        implements java.io.Serializable {
    
        private static final long serialVersionUID = 7373984972572414691L;
    
        /**
         * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance
         * with initial synchronization state of zero.
         */
        protected AbstractQueuedSynchronizer() { }
    
        /**
         * Wait queue node class.
         *
         * <p>The wait queue is a variant of a "CLH" (Craig, Landin, and
         * Hagersten) lock queue. CLH locks are normally used for
         * spinlocks.  We instead use them for blocking synchronizers, but
         * use the same basic tactic of holding some of the control
         * information about a thread in the predecessor of its node.  A
         * "status" field in each node keeps track of whether a thread
         * should block.  A node is signalled when its predecessor
         * releases.  Each node of the queue otherwise serves as a
         * specific-notification-style monitor holding a single waiting
         * thread. The status field does NOT control whether threads are
         * granted locks etc though.  A thread may try to acquire if it is
         * first in the queue. But being first does not guarantee success;
         * it only gives the right to contend.  So the currently released
         * contender thread may need to rewait.
         *
         * <p>To enqueue into a CLH lock, you atomically splice it in as new
         * tail. To dequeue, you just set the head field.
         * <pre>
         *      +------+  prev +-----+       +-----+
         * head |      | <---- |     | <---- |     |  tail
         *      +------+       +-----+       +-----+
         * </pre>
         *
         * <p>Insertion into a CLH queue requires only a single atomic
         * operation on "tail", so there is a simple atomic point of
         * demarcation from unqueued to queued. Similarly, dequeing
         * involves only updating the "head". However, it takes a bit
         * more work for nodes to determine who their successors are,
         * in part to deal with possible cancellation due to timeouts
         * and interrupts.
         *
         * <p>The "prev" links (not used in original CLH locks), are mainly
         * needed to handle cancellation. If a node is cancelled, its
         * successor is (normally) relinked to a non-cancelled
         * predecessor. For explanation of similar mechanics in the case
         * of spin locks, see the papers by Scott and Scherer at
         * http://www.cs.rochester.edu/u/scott/synchronization/
         *
         * <p>We also use "next" links to implement blocking mechanics.
         * The thread id for each node is kept in its own node, so a
         * predecessor signals the next node to wake up by traversing
         * next link to determine which thread it is.  Determination of
         * successor must avoid races with newly queued nodes to set
         * the "next" fields of their predecessors.  This is solved
         * when necessary by checking backwards from the atomically
         * updated "tail" when a node's successor appears to be null.
         * (Or, said differently, the next-links are an optimization
         * so that we don't usually need a backward scan.)
         *
         * <p>Cancellation introduces some conservatism to the basic
         * algorithms.  Since we must poll for cancellation of other
         * nodes, we can miss noticing whether a cancelled node is
         * ahead or behind us. This is dealt with by always unparking
         * successors upon cancellation, allowing them to stabilize on
         * a new predecessor, unless we can identify an uncancelled
         * predecessor who will carry this responsibility.
         *
         * <p>CLH queues need a dummy header node to get started. But
         * we don't create them on construction, because it would be wasted
         * effort if there is never contention. Instead, the node
         * is constructed and head and tail pointers are set upon first
         * contention.
         *
         * <p>Threads waiting on Conditions use the same nodes, but
         * use an additional link. Conditions only need to link nodes
         * in simple (non-concurrent) linked queues because they are
         * only accessed when exclusively held.  Upon await, a node is
         * inserted into a condition queue.  Upon signal, the node is
         * transferred to the main queue.  A special value of status
         * field is used to mark which queue a node is on.
         *
         * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill
         * Scherer and Michael Scott, along with members of JSR-166
         * expert group, for helpful ideas, discussions, and critiques
         * on the design of this class.
         */
        static final class Node {
            /** Marker to indicate a node is waiting in shared mode */
            static final Node SHARED = new Node();
            /** Marker to indicate a node is waiting in exclusive mode */
            static final Node EXCLUSIVE = null;
    
            /** waitStatus value to indicate thread has cancelled */
            static final int CANCELLED =  1;
            /** waitStatus value to indicate successor's thread needs unparking */
            static final int SIGNAL    = -1;
            /** waitStatus value to indicate thread is waiting on condition */
            static final int CONDITION = -2;
            /**
             * waitStatus value to indicate the next acquireShared should
             * unconditionally propagate
             */
            static final int PROPAGATE = -3;
    
            /**
             * Status field, taking on only the values:
             *   SIGNAL:     The successor of this node is (or will soon be)
             *               blocked (via park), so the current node must
             *               unpark its successor when it releases or
             *               cancels. To avoid races, acquire methods must
             *               first indicate they need a signal,
             *               then retry the atomic acquire, and then,
             *               on failure, block.
             *   CANCELLED:  This node is cancelled due to timeout or interrupt.
             *               Nodes never leave this state. In particular,
             *               a thread with cancelled node never again blocks.
             *   CONDITION:  This node is currently on a condition queue.
             *               It will not be used as a sync queue node
             *               until transferred, at which time the status
             *               will be set to 0. (Use of this value here has
             *               nothing to do with the other uses of the
             *               field, but simplifies mechanics.)
             *   PROPAGATE:  A releaseShared should be propagated to other
             *               nodes. This is set (for head node only) in
             *               doReleaseShared to ensure propagation
             *               continues, even if other operations have
             *               since intervened.
             *   0:          None of the above
             *
             * The values are arranged numerically to simplify use.
             * Non-negative values mean that a node doesn't need to
             * signal. So, most code doesn't need to check for particular
             * values, just for sign.
             *
             * The field is initialized to 0 for normal sync nodes, and
             * CONDITION for condition nodes.  It is modified using CAS
             * (or when possible, unconditional volatile writes).
             */
            volatile int waitStatus;
    
            /**
             * Link to predecessor node that current node/thread relies on
             * for checking waitStatus. Assigned during enqueing, and nulled
             * out (for sake of GC) only upon dequeuing.  Also, upon
             * cancellation of a predecessor, we short-circuit while
             * finding a non-cancelled one, which will always exist
             * because the head node is never cancelled: A node becomes
             * head only as a result of successful acquire. A
             * cancelled thread never succeeds in acquiring, and a thread only
             * cancels itself, not any other node.
             */
            volatile Node prev;
    
            /**
             * Link to the successor node that the current node/thread
             * unparks upon release. Assigned during enqueuing, adjusted
             * when bypassing cancelled predecessors, and nulled out (for
             * sake of GC) when dequeued.  The enq operation does not
             * assign next field of a predecessor until after attachment,
             * so seeing a null next field does not necessarily mean that
             * node is at end of queue. However, if a next field appears
             * to be null, we can scan prev's from the tail to
             * double-check.  The next field of cancelled nodes is set to
             * point to the node itself instead of null, to make life
             * easier for isOnSyncQueue.
             */
            volatile Node next;
    
            /**
             * The thread that enqueued this node.  Initialized on
             * construction and nulled out after use.
             */
            volatile Thread thread;
    
            /**
             * Link to next node waiting on condition, or the special
             * value SHARED.  Because condition queues are accessed only
             * when holding in exclusive mode, we just need a simple
             * linked queue to hold nodes while they are waiting on
             * conditions. They are then transferred to the queue to
             * re-acquire. And because conditions can only be exclusive,
             * we save a field by using special value to indicate shared
             * mode.
             */
            Node nextWaiter;
    
            /**
             * Returns true if node is waiting in shared mode
             */
            final boolean isShared() {
                return nextWaiter == SHARED;
            }
    
            /**
             * Returns previous node, or throws NullPointerException if null.
             * Use when predecessor cannot be null.  The null check could
             * be elided, but is present to help the VM.
             *
             * @return the predecessor of this node
             */
            final Node predecessor() throws NullPointerException {
                Node p = prev;
                if (p == null)
                    throw new NullPointerException();
                else
                    return p;
            }
    
            Node() {    // Used to establish initial head or SHARED marker
            }
    
            Node(Thread thread, Node mode) {     // Used by addWaiter
                this.nextWaiter = mode;
                this.thread = thread;
            }
    
            Node(Thread thread, int waitStatus) { // Used by Condition
                this.waitStatus = waitStatus;
                this.thread = thread;
            }
        }
    
        /**
         * Head of the wait queue, lazily initialized.  Except for
         * initialization, it is modified only via method setHead.  Note:
         * If head exists, its waitStatus is guaranteed not to be
         * CANCELLED.
         */
        private transient volatile Node head;
    
        /**
         * Tail of the wait queue, lazily initialized.  Modified only via
         * method enq to add new wait node.
         */
        private transient volatile Node tail;
    
        /**
         * The synchronization state.
         */
        private volatile int state;
    
        /**
         * Returns the current value of synchronization state.
         * This operation has memory semantics of a <tt>volatile</tt> read.
         * @return current state value
         */
        protected final int getState() {
            return state;
        }
    
        /**
         * Sets the value of synchronization state.
         * This operation has memory semantics of a <tt>volatile</tt> write.
         * @param newState the new state value
         */
        protected final void setState(int newState) {
            state = newState;
        }
    
        /**
         * Atomically sets synchronization state to the given updated
         * value if the current state value equals the expected value.
         * This operation has memory semantics of a <tt>volatile</tt> read
         * and write.
         *
         * @param expect the expected value
         * @param update the new value
         * @return true if successful. False return indicates that the actual
         *         value was not equal to the expected value.
         */
        protected final boolean compareAndSetState(int expect, int update) {
            // See below for intrinsics setup to support this
            return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
        }
    
        // Queuing utilities
    
        /**
         * The number of nanoseconds for which it is faster to spin
         * rather than to use timed park. A rough estimate suffices
         * to improve responsiveness with very short timeouts.
         */
        static final long spinForTimeoutThreshold = 1000L;
    
        /**
         * Inserts node into queue, initializing if necessary. See picture above.
         * @param node the node to insert
         * @return node's predecessor
         */
        private Node enq(final Node node) {
            for (;;) {
                Node t = tail;
                if (t == null) { // Must initialize
                    if (compareAndSetHead(new Node()))
                        tail = head;
                } else {
                    node.prev = t;
                    if (compareAndSetTail(t, node)) {
                        t.next = node;
                        return t;
                    }
                }
            }
        }
    
        /**
         * Creates and enqueues node for current thread and given mode.
         *
         * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
         * @return the new node
         */
        private Node addWaiter(Node mode) {
            Node node = new Node(Thread.currentThread(), mode);
            // Try the fast path of enq; backup to full enq on failure
            Node pred = tail;
            if (pred != null) {
                node.prev = pred;
                if (compareAndSetTail(pred, node)) {
                    pred.next = node;
                    return node;
                }
            }
            enq(node);
            return node;
        }
    
        /**
         * Sets head of queue to be node, thus dequeuing. Called only by
         * acquire methods.  Also nulls out unused fields for sake of GC
         * and to suppress unnecessary signals and traversals.
         *
         * @param node the node
         */
        private void setHead(Node node) {
            head = node;
            node.thread = null;
            node.prev = null;
        }
    
        /**
         * Wakes up node's successor, if one exists.
         *
         * @param node the node
         */
        private void unparkSuccessor(Node node) {
            /*
             * If status is negative (i.e., possibly needing signal) try
             * to clear in anticipation of signalling.  It is OK if this
             * fails or if status is changed by waiting thread.
             */
            int ws = node.waitStatus;
            if (ws < 0)
                compareAndSetWaitStatus(node, ws, 0);
    
            /*
             * Thread to unpark is held in successor, which is normally
             * just the next node.  But if cancelled or apparently null,
             * traverse backwards from tail to find the actual
             * non-cancelled successor.
             */
            Node s = node.next;
            if (s == null || s.waitStatus > 0) {
                s = null;
                for (Node t = tail; t != null && t != node; t = t.prev)
                    if (t.waitStatus <= 0)
                        s = t;
            }
            if (s != null)
                LockSupport.unpark(s.thread);
        }
    
        /**
         * Release action for shared mode -- signal successor and ensure
         * propagation. (Note: For exclusive mode, release just amounts
         * to calling unparkSuccessor of head if it needs signal.)
         */
        private void doReleaseShared() {
            /*
             * Ensure that a release propagates, even if there are other
             * in-progress acquires/releases.  This proceeds in the usual
             * way of trying to unparkSuccessor of head if it needs
             * signal. But if it does not, status is set to PROPAGATE to
             * ensure that upon release, propagation continues.
             * Additionally, we must loop in case a new node is added
             * while we are doing this. Also, unlike other uses of
             * unparkSuccessor, we need to know if CAS to reset status
             * fails, if so rechecking.
             */
            for (;;) {
                Node h = head;
                if (h != null && h != tail) {
                    int ws = h.waitStatus;
                    if (ws == Node.SIGNAL) {
                        if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                            continue;            // loop to recheck cases
                        unparkSuccessor(h);
                    }
                    else if (ws == 0 &&
                             !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                        continue;                // loop on failed CAS
                }
                if (h == head)                   // loop if head changed
                    break;
            }
        }
    
        /**
         * Sets head of queue, and checks if successor may be waiting
         * in shared mode, if so propagating if either propagate > 0 or
         * PROPAGATE status was set.
         *
         * @param node the node
         * @param propagate the return value from a tryAcquireShared
         */
        private void setHeadAndPropagate(Node node, int propagate) {
            Node h = head; // Record old head for check below
            setHead(node);
            /*
             * Try to signal next queued node if:
             *   Propagation was indicated by caller,
             *     or was recorded (as h.waitStatus) by a previous operation
             *     (note: this uses sign-check of waitStatus because
             *      PROPAGATE status may transition to SIGNAL.)
             * and
             *   The next node is waiting in shared mode,
             *     or we don't know, because it appears null
             *
             * The conservatism in both of these checks may cause
             * unnecessary wake-ups, but only when there are multiple
             * racing acquires/releases, so most need signals now or soon
             * anyway.
             */
            if (propagate > 0 || h == null || h.waitStatus < 0) {
                Node s = node.next;
                if (s == null || s.isShared())
                    doReleaseShared();
            }
        }
    
        // Utilities for various versions of acquire
    
        /**
         * Cancels an ongoing attempt to acquire.
         *
         * @param node the node
         */
        private void cancelAcquire(Node node) {
            // Ignore if node doesn't exist
            if (node == null)
                return;
    
            node.thread = null;
    
            // Skip cancelled predecessors
            Node pred = node.prev;
            while (pred.waitStatus > 0)
                node.prev = pred = pred.prev;
    
            // predNext is the apparent node to unsplice. CASes below will
            // fail if not, in which case, we lost race vs another cancel
            // or signal, so no further action is necessary.
            Node predNext = pred.next;
    
            // Can use unconditional write instead of CAS here.
            // After this atomic step, other Nodes can skip past us.
            // Before, we are free of interference from other threads.
            node.waitStatus = Node.CANCELLED;
    
            // If we are the tail, remove ourselves.
            if (node == tail && compareAndSetTail(node, pred)) {
                compareAndSetNext(pred, predNext, null);
            } else {
                // If successor needs signal, try to set pred's next-link
                // so it will get one. Otherwise wake it up to propagate.
                int ws;
                if (pred != head &&
                    ((ws = pred.waitStatus) == Node.SIGNAL ||
                     (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                    pred.thread != null) {
                    Node next = node.next;
                    if (next != null && next.waitStatus <= 0)
                        compareAndSetNext(pred, predNext, next);
                } else {
                    unparkSuccessor(node);
                }
    
                node.next = node; // help GC
            }
        }
    
        /**
         * Checks and updates status for a node that failed to acquire.
         * Returns true if thread should block. This is the main signal
         * control in all acquire loops.  Requires that pred == node.prev
         *
         * @param pred node's predecessor holding status
         * @param node the node
         * @return {@code true} if thread should block
         */
        private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
            int ws = pred.waitStatus;
            if (ws == Node.SIGNAL)
                /*
                 * This node has already set status asking a release
                 * to signal it, so it can safely park.
                 */
                return true;
            if (ws > 0) {
                /*
                 * Predecessor was cancelled. Skip over predecessors and
                 * indicate retry.
                 */
                do {
                    node.prev = pred = pred.prev;
                } while (pred.waitStatus > 0);
                pred.next = node;
            } else {
                /*
                 * waitStatus must be 0 or PROPAGATE.  Indicate that we
                 * need a signal, but don't park yet.  Caller will need to
                 * retry to make sure it cannot acquire before parking.
                 */
                compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
            }
            return false;
        }
    
        /**
         * Convenience method to interrupt current thread.
         */
        private static void selfInterrupt() {
            Thread.currentThread().interrupt();
        }
    
        /**
         * Convenience method to park and then check if interrupted
         *
         * @return {@code true} if interrupted
         */
        private final boolean parkAndCheckInterrupt() {
            LockSupport.park(this);
            return Thread.interrupted();
        }
    
        /*
         * Various flavors of acquire, varying in exclusive/shared and
         * control modes.  Each is mostly the same, but annoyingly
         * different.  Only a little bit of factoring is possible due to
         * interactions of exception mechanics (including ensuring that we
         * cancel if tryAcquire throws exception) and other control, at
         * least not without hurting performance too much.
         */
    
        /**
         * Acquires in exclusive uninterruptible mode for thread already in
         * queue. Used by condition wait methods as well as acquire.
         *
         * @param node the node
         * @param arg the acquire argument
         * @return {@code true} if interrupted while waiting
         */
        final boolean acquireQueued(final Node node, int arg) {
            boolean failed = true;
            try {
                boolean interrupted = false;
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head && tryAcquire(arg)) {
                        setHead(node);
                        p.next = null; // help GC
                        failed = false;
                        return interrupted;
                    }
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                        interrupted = true;
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        /**
         * Acquires in exclusive interruptible mode.
         * @param arg the acquire argument
         */
        private void doAcquireInterruptibly(int arg)
            throws InterruptedException {
            final Node node = addWaiter(Node.EXCLUSIVE);
            boolean failed = true;
            try {
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head && tryAcquire(arg)) {
                        setHead(node);
                        p.next = null; // help GC
                        failed = false;
                        return;
                    }
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        /**
         * Acquires in exclusive timed mode.
         *
         * @param arg the acquire argument
         * @param nanosTimeout max wait time
         * @return {@code true} if acquired
         */
        private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
            long lastTime = System.nanoTime();
            final Node node = addWaiter(Node.EXCLUSIVE);
            boolean failed = true;
            try {
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head && tryAcquire(arg)) {
                        setHead(node);
                        p.next = null; // help GC
                        failed = false;
                        return true;
                    }
                    if (nanosTimeout <= 0)
                        return false;
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold)
                        LockSupport.parkNanos(this, nanosTimeout);
                    long now = System.nanoTime();
                    nanosTimeout -= now - lastTime;
                    lastTime = now;
                    if (Thread.interrupted())
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        /**
         * Acquires in shared uninterruptible mode.
         * @param arg the acquire argument
         */
        private void doAcquireShared(int arg) {
            final Node node = addWaiter(Node.SHARED);
            boolean failed = true;
            try {
                boolean interrupted = false;
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head) {
                        int r = tryAcquireShared(arg);
                        if (r >= 0) {
                            setHeadAndPropagate(node, r);
                            p.next = null; // help GC
                            if (interrupted)
                                selfInterrupt();
                            failed = false;
                            return;
                        }
                    }
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                        interrupted = true;
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        /**
         * Acquires in shared interruptible mode.
         * @param arg the acquire argument
         */
        private void doAcquireSharedInterruptibly(int arg)
            throws InterruptedException {
            final Node node = addWaiter(Node.SHARED);
            boolean failed = true;
            try {
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head) {
                        int r = tryAcquireShared(arg);
                        if (r >= 0) {
                            setHeadAndPropagate(node, r);
                            p.next = null; // help GC
                            failed = false;
                            return;
                        }
                    }
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        /**
         * Acquires in shared timed mode.
         *
         * @param arg the acquire argument
         * @param nanosTimeout max wait time
         * @return {@code true} if acquired
         */
        private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
    
            long lastTime = System.nanoTime();
            final Node node = addWaiter(Node.SHARED);
            boolean failed = true;
            try {
                for (;;) {
                    final Node p = node.predecessor();
                    if (p == head) {
                        int r = tryAcquireShared(arg);
                        if (r >= 0) {
                            setHeadAndPropagate(node, r);
                            p.next = null; // help GC
                            failed = false;
                            return true;
                        }
                    }
                    if (nanosTimeout <= 0)
                        return false;
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold)
                        LockSupport.parkNanos(this, nanosTimeout);
                    long now = System.nanoTime();
                    nanosTimeout -= now - lastTime;
                    lastTime = now;
                    if (Thread.interrupted())
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }
    
        // Main exported methods
    
        /**
         * Attempts to acquire in exclusive mode. This method should query
         * if the state of the object permits it to be acquired in the
         * exclusive mode, and if so to acquire it.
         *
         * <p>This method is always invoked by the thread performing
         * acquire.  If this method reports failure, the acquire method
         * may queue the thread, if it is not already queued, until it is
         * signalled by a release from some other thread. This can be used
         * to implement method {@link Lock#tryLock()}.
         *
         * <p>The default
         * implementation throws {@link UnsupportedOperationException}.
         *
         * @param arg the acquire argument. This value is always the one
         *        passed to an acquire method, or is the value saved on entry
         *        to a condition wait.  The value is otherwise uninterpreted
         *        and can represent anything you like.
         * @return {@code true} if successful. Upon success, this object has
         *         been acquired.
         * @throws IllegalMonitorStateException if acquiring would place this
         *         synchronizer in an illegal state. This exception must be
         *         thrown in a consistent fashion for synchronization to work
         *         correctly.
         * @throws UnsupportedOperationException if exclusive mode is not supported
         */
        protected boolean tryAcquire(int arg) {
            throw new UnsupportedOperationException();
        }
    
        /**
         * Attempts to set the state to reflect a release in exclusive
         * mode.
         *
         * <p>This method is always invoked by the thread performing release.
         *
         * <p>The default implementation throws
         * {@link UnsupportedOperationException}.
         *
         * @param arg the release argument. This value is always the one
         *        passed to a release method, or the current state value upon
         *        entry to a condition wait.  The value is otherwise
         *        uninterpreted and can represent anything you like.
         * @return {@code true} if this object is now in a fully released
         *         state, so that any waiting threads may attempt to acquire;
         *         and {@code false} otherwise.
         * @throws IllegalMonitorStateException if releasing would place this
         *         synchronizer in an illegal state. This exception must be
         *         thrown in a consistent fashion for synchronization to work
         *         correctly.
         * @throws UnsupportedOperationException if exclusive mode is not supported
         */
        protected boolean tryRelease(int arg) {
            throw new UnsupportedOperationException();
        }
    
        /**
         * Attempts to acquire in shared mode. This method should query if
         * the state of the object permits it to be acquired in the shared
         * mode, and if so to acquire it.
         *
         * <p>This method is always invoked by the thread performing
         * acquire.  If this method reports failure, the acquire method
         * may queue the thread, if it is not already queued, until it is
         * signalled by a release from some other thread.
         *
         * <p>The default implementation throws {@link
         * UnsupportedOperationException}.
         *
         * @param arg the acquire argument. This value is always the one
         *        passed to an acquire method, or is the value saved on entry
         *        to a condition wait.  The value is otherwise uninterpreted
         *        and can represent anything you like.
         * @return a negative value on failure; zero if acquisition in shared
         *         mode succeeded but no subsequent shared-mode acquire can
         *         succeed; and a positive value if acquisition in shared
         *         mode succeeded and subsequent shared-mode acquires might
         *         also succeed, in which case a subsequent waiting thread
         *         must check availability. (Support for three different
         *         return values enables this method to be used in contexts
         *         where acquires only sometimes act exclusively.)  Upon
         *         success, this object has been acquired.
         * @throws IllegalMonitorStateException if acquiring would place this
         *         synchronizer in an illegal state. This exception must be
         *         thrown in a consistent fashion for synchronization to work
         *         correctly.
         * @throws UnsupportedOperationException if shared mode is not supported
         */
        protected int tryAcquireShared(int arg) {
            throw new UnsupportedOperationException();
        }
    
        /**
         * Attempts to set the state to reflect a release in shared mode.
         *
         * <p>This method is always invoked by the thread performing release.
         *
         * <p>The default implementation throws
         * {@link UnsupportedOperationException}.
         *
         * @param arg the release argument. This value is always the one
         *        passed to a release method, or the current state value upon
         *        entry to a condition wait.  The value is otherwise
         *        uninterpreted and can represent anything you like.
         * @return {@code true} if this release of shared mode may permit a
         *         waiting acquire (shared or exclusive) to succeed; and
         *         {@code false} otherwise
         * @throws IllegalMonitorStateException if releasing would place this
         *         synchronizer in an illegal state. This exception must be
         *         thrown in a consistent fashion for synchronization to work
         *         correctly.
         * @throws UnsupportedOperationException if shared mode is not supported
         */
        protected boolean tryReleaseShared(int arg) {
            throw new UnsupportedOperationException();
        }
    
        /**
         * Returns {@code true} if synchronization is held exclusively with
         * respect to the current (calling) thread.  This method is invoked
         * upon each call to a non-waiting {@link ConditionObject} method.
         * (Waiting methods instead invoke {@link #release}.)
         *
         * <p>The default implementation throws {@link
         * UnsupportedOperationException}. This method is invoked
         * internally only within {@link ConditionObject} methods, so need
         * not be defined if conditions are not used.
         *
         * @return {@code true} if synchronization is held exclusively;
         *         {@code false} otherwise
         * @throws UnsupportedOperationException if conditions are not supported
         */
        protected boolean isHeldExclusively() {
            throw new UnsupportedOperationException();
        }
    
        /**
         * Acquires in exclusive mode, ignoring interrupts.  Implemented
         * by invoking at least once {@link #tryAcquire},
         * returning on success.  Otherwise the thread is queued, possibly
         * repeatedly blocking and unblocking, invoking {@link
         * #tryAcquire} until success.  This method can be used
         * to implement method {@link Lock#lock}.
         *
         * @param arg the acquire argument.  This value is conveyed to
         *        {@link #tryAcquire} but is otherwise uninterpreted and
         *        can represent anything you like.
         */
        public final void acquire(int arg) {
            if (!tryAcquire(arg) &&
                acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
                selfInterrupt();
        }
    
        /**
         * Acquires in exclusive mode, aborting if interrupted.
         * Implemented by first checking interrupt status, then invoking
         * at least once {@link #tryAcquire}, returning on
         * success.  Otherwise the thread is queued, possibly repeatedly
         * blocking and unblocking, invoking {@link #tryAcquire}
         * until success or the thread is interrupted.  This method can be
         * used to implement method {@link Lock#lockInterruptibly}.
         *
         * @param arg the acquire argument.  This value is conveyed to
         *        {@link #tryAcquire} but is otherwise uninterpreted and
         *        can represent anything you like.
         * @throws InterruptedException if the current thread is interrupted
         */
        public final void acquireInterruptibly(int arg)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            if (!tryAcquire(arg))
                doAcquireInterruptibly(arg);
        }
    
        /**
         * Attempts to acquire in exclusive mode, aborting if interrupted,
         * and failing if the given timeout elapses.  Implemented by first
         * checking interrupt status, then invoking at least once {@link
         * #tryAcquire}, returning on success.  Otherwise, the thread is
         * queued, possibly repeatedly blocking and unblocking, invoking
         * {@link #tryAcquire} until success or the thread is interrupted
         * or the timeout elapses.  This method can be used to implement
         * method {@link Lock#tryLock(long, TimeUnit)}.
         *
         * @param arg the acquire argument.  This value is conveyed to
         *        {@link #tryAcquire} but is otherwise uninterpreted and
         *        can represent anything you like.
         * @param nanosTimeout the maximum number of nanoseconds to wait
         * @return {@code true} if acquired; {@code false} if timed out
         * @throws InterruptedException if the current thread is interrupted
         */
        public final boolean tryAcquireNanos(int arg, long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            return tryAcquire(arg) ||
                doAcquireNanos(arg, nanosTimeout);
        }
    
        /**
         * Releases in exclusive mode.  Implemented by unblocking one or
         * more threads if {@link #tryRelease} returns true.
         * This method can be used to implement method {@link Lock#unlock}.
         *
         * @param arg the release argument.  This value is conveyed to
         *        {@link #tryRelease} but is otherwise uninterpreted and
         *        can represent anything you like.
         * @return the value returned from {@link #tryRelease}
         */
        public final boolean release(int arg) {
            if (tryRelease(arg)) {
                Node h = head;
                if (h != null && h.waitStatus != 0)
                    unparkSuccessor(h);
                return true;
            }
            return false;
        }
    
        /**
         * Acquires in shared mode, ignoring interrupts.  Implemented by
         * first invoking at least once {@link #tryAcquireShared},
         * returning on success.  Otherwise the thread is queued, possibly
         * repeatedly blocking and unblocking, invoking {@link
         * #tryAcquireShared} until success.
         *
         * @param arg the acquire argument.  This value is conveyed to
         *        {@link #tryAcquireShared} but is otherwise uninterpreted
         *        and can represent anything you like.
         */
        public final void acquireShared(int arg) {
            if (tryAcquireShared(arg) < 0)
                doAcquireShared(arg);
        }
    
        /**
         * Acquires in shared mode, aborting if interrupted.  Implemented
         * by first checking interrupt status, then invoking at least once
         * {@link #tryAcquireShared}, returning on success.  Otherwise the
         * thread is queued, possibly repeatedly blocking and unblocking,
         * invoking {@link #tryAcquireShared} until success or the thread
         * is interrupted.
         * @param arg the acquire argument
         * This value is conveyed to {@link #tryAcquireShared} but is
         * otherwise uninterpreted and can represent anything
         * you like.
         * @throws InterruptedException if the current thread is interrupted
         */
        public final void acquireSharedInterruptibly(int arg)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            if (tryAcquireShared(arg) < 0)
                doAcquireSharedInterruptibly(arg);
        }
    
        /**
         * Attempts to acquire in shared mode, aborting if interrupted, and
         * failing if the given timeout elapses.  Implemented by first
         * checking interrupt status, then invoking at least once {@link
         * #tryAcquireShared}, returning on success.  Otherwise, the
         * thread is queued, possibly repeatedly blocking and unblocking,
         * invoking {@link #tryAcquireShared} until success or the thread
         * is interrupted or the timeout elapses.
         *
         * @param arg the acquire argument.  This value is conveyed to
         *        {@link #tryAcquireShared} but is otherwise uninterpreted
         *        and can represent anything you like.
         * @param nanosTimeout the maximum number of nanoseconds to wait
         * @return {@code true} if acquired; {@code false} if timed out
         * @throws InterruptedException if the current thread is interrupted
         */
        public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            return tryAcquireShared(arg) >= 0 ||
                doAcquireSharedNanos(arg, nanosTimeout);
        }
    
        /**
         * Releases in shared mode.  Implemented by unblocking one or more
         * threads if {@link #tryReleaseShared} returns true.
         *
         * @param arg the release argument.  This value is conveyed to
         *        {@link #tryReleaseShared} but is otherwise uninterpreted
         *        and can represent anything you like.
         * @return the value returned from {@link #tryReleaseShared}
         */
        public final boolean releaseShared(int arg) {
            if (tryReleaseShared(arg)) {
                doReleaseShared();
                return true;
            }
            return false;
        }
    
        // Queue inspection methods
    
        /**
         * Queries whether any threads are waiting to acquire. Note that
         * because cancellations due to interrupts and timeouts may occur
         * at any time, a {@code true} return does not guarantee that any
         * other thread will ever acquire.
         *
         * <p>In this implementation, this operation returns in
         * constant time.
         *
         * @return {@code true} if there may be other threads waiting to acquire
         */
        public final boolean hasQueuedThreads() {
            return head != tail;
        }
    
        /**
         * Queries whether any threads have ever contended to acquire this
         * synchronizer; that is if an acquire method has ever blocked.
         *
         * <p>In this implementation, this operation returns in
         * constant time.
         *
         * @return {@code true} if there has ever been contention
         */
        public final boolean hasContended() {
            return head != null;
        }
    
        /**
         * Returns the first (longest-waiting) thread in the queue, or
         * {@code null} if no threads are currently queued.
         *
         * <p>In this implementation, this operation normally returns in
         * constant time, but may iterate upon contention if other threads are
         * concurrently modifying the queue.
         *
         * @return the first (longest-waiting) thread in the queue, or
         *         {@code null} if no threads are currently queued
         */
        public final Thread getFirstQueuedThread() {
            // handle only fast path, else relay
            return (head == tail) ? null : fullGetFirstQueuedThread();
        }
    
        /**
         * Version of getFirstQueuedThread called when fastpath fails
         */
        private Thread fullGetFirstQueuedThread() {
            /*
             * The first node is normally head.next. Try to get its
             * thread field, ensuring consistent reads: If thread
             * field is nulled out or s.prev is no longer head, then
             * some other thread(s) concurrently performed setHead in
             * between some of our reads. We try this twice before
             * resorting to traversal.
             */
            Node h, s;
            Thread st;
            if (((h = head) != null && (s = h.next) != null &&
                 s.prev == head && (st = s.thread) != null) ||
                ((h = head) != null && (s = h.next) != null &&
                 s.prev == head && (st = s.thread) != null))
                return st;
    
            /*
             * Head's next field might not have been set yet, or may have
             * been unset after setHead. So we must check to see if tail
             * is actually first node. If not, we continue on, safely
             * traversing from tail back to head to find first,
             * guaranteeing termination.
             */
    
            Node t = tail;
            Thread firstThread = null;
            while (t != null && t != head) {
                Thread tt = t.thread;
                if (tt != null)
                    firstThread = tt;
                t = t.prev;
            }
            return firstThread;
        }
    
        /**
         * Returns true if the given thread is currently queued.
         *
         * <p>This implementation traverses the queue to determine
         * presence of the given thread.
         *
         * @param thread the thread
         * @return {@code true} if the given thread is on the queue
         * @throws NullPointerException if the thread is null
         */
        public final boolean isQueued(Thread thread) {
            if (thread == null)
                throw new NullPointerException();
            for (Node p = tail; p != null; p = p.prev)
                if (p.thread == thread)
                    return true;
            return false;
        }
    
        /**
         * Returns {@code true} if the apparent first queued thread, if one
         * exists, is waiting in exclusive mode.  If this method returns
         * {@code true}, and the current thread is attempting to acquire in
         * shared mode (that is, this method is invoked from {@link
         * #tryAcquireShared}) then it is guaranteed that the current thread
         * is not the first queued thread.  Used only as a heuristic in
         * ReentrantReadWriteLock.
         */
        final boolean apparentlyFirstQueuedIsExclusive() {
            Node h, s;
            return (h = head) != null &&
                (s = h.next)  != null &&
                !s.isShared()         &&
                s.thread != null;
        }
    
        /**
         * Queries whether any threads have been waiting to acquire longer
         * than the current thread.
         *
         * <p>An invocation of this method is equivalent to (but may be
         * more efficient than):
         *  <pre> {@code
         * getFirstQueuedThread() != Thread.currentThread() &&
         * hasQueuedThreads()}</pre>
         *
         * <p>Note that because cancellations due to interrupts and
         * timeouts may occur at any time, a {@code true} return does not
         * guarantee that some other thread will acquire before the current
         * thread.  Likewise, it is possible for another thread to win a
         * race to enqueue after this method has returned {@code false},
         * due to the queue being empty.
         *
         * <p>This method is designed to be used by a fair synchronizer to
         * avoid <a href="AbstractQueuedSynchronizer#barging">barging</a>.
         * Such a synchronizer's {@link #tryAcquire} method should return
         * {@code false}, and its {@link #tryAcquireShared} method should
         * return a negative value, if this method returns {@code true}
         * (unless this is a reentrant acquire).  For example, the {@code
         * tryAcquire} method for a fair, reentrant, exclusive mode
         * synchronizer might look like this:
         *
         *  <pre> {@code
         * protected boolean tryAcquire(int arg) {
         *   if (isHeldExclusively()) {
         *     // A reentrant acquire; increment hold count
         *     return true;
         *   } else if (hasQueuedPredecessors()) {
         *     return false;
         *   } else {
         *     // try to acquire normally
         *   }
         * }}</pre>
         *
         * @return {@code true} if there is a queued thread preceding the
         *         current thread, and {@code false} if the current thread
         *         is at the head of the queue or the queue is empty
         * @since 1.7
         */
        public final boolean hasQueuedPredecessors() {
            // The correctness of this depends on head being initialized
            // before tail and on head.next being accurate if the current
            // thread is first in queue.
            Node t = tail; // Read fields in reverse initialization order
            Node h = head;
            Node s;
            return h != t &&
                ((s = h.next) == null || s.thread != Thread.currentThread());
        }
    
    
        // Instrumentation and monitoring methods
    
        /**
         * Returns an estimate of the number of threads waiting to
         * acquire.  The value is only an estimate because the number of
         * threads may change dynamically while this method traverses
         * internal data structures.  This method is designed for use in
         * monitoring system state, not for synchronization
         * control.
         *
         * @return the estimated number of threads waiting to acquire
         */
        public final int getQueueLength() {
            int n = 0;
            for (Node p = tail; p != null; p = p.prev) {
                if (p.thread != null)
                    ++n;
            }
            return n;
        }
    
        /**
         * Returns a collection containing threads that may be waiting to
         * acquire.  Because the actual set of threads may change
         * dynamically while constructing this result, the returned
         * collection is only a best-effort estimate.  The elements of the
         * returned collection are in no particular order.  This method is
         * designed to facilitate construction of subclasses that provide
         * more extensive monitoring facilities.
         *
         * @return the collection of threads
         */
        public final Collection<Thread> getQueuedThreads() {
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node p = tail; p != null; p = p.prev) {
                Thread t = p.thread;
                if (t != null)
                    list.add(t);
            }
            return list;
        }
    
        /**
         * Returns a collection containing threads that may be waiting to
         * acquire in exclusive mode. This has the same properties
         * as {@link #getQueuedThreads} except that it only returns
         * those threads waiting due to an exclusive acquire.
         *
         * @return the collection of threads
         */
        public final Collection<Thread> getExclusiveQueuedThreads() {
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node p = tail; p != null; p = p.prev) {
                if (!p.isShared()) {
                    Thread t = p.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    
        /**
         * Returns a collection containing threads that may be waiting to
         * acquire in shared mode. This has the same properties
         * as {@link #getQueuedThreads} except that it only returns
         * those threads waiting due to a shared acquire.
         *
         * @return the collection of threads
         */
        public final Collection<Thread> getSharedQueuedThreads() {
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node p = tail; p != null; p = p.prev) {
                if (p.isShared()) {
                    Thread t = p.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    
        /**
         * Returns a string identifying this synchronizer, as well as its state.
         * The state, in brackets, includes the String {@code "State ="}
         * followed by the current value of {@link #getState}, and either
         * {@code "nonempty"} or {@code "empty"} depending on whether the
         * queue is empty.
         *
         * @return a string identifying this synchronizer, as well as its state
         */
        public String toString() {
            int s = getState();
            String q  = hasQueuedThreads() ? "non" : "";
            return super.toString() +
                "[State = " + s + ", " + q + "empty queue]";
        }
    
    
        // Internal support methods for Conditions
    
        /**
         * Returns true if a node, always one that was initially placed on
         * a condition queue, is now waiting to reacquire on sync queue.
         * @param node the node
         * @return true if is reacquiring
         */
        final boolean isOnSyncQueue(Node node) {
            if (node.waitStatus == Node.CONDITION || node.prev == null)
                return false;
            if (node.next != null) // If has successor, it must be on queue
                return true;
            /*
             * node.prev can be non-null, but not yet on queue because
             * the CAS to place it on queue can fail. So we have to
             * traverse from tail to make sure it actually made it.  It
             * will always be near the tail in calls to this method, and
             * unless the CAS failed (which is unlikely), it will be
             * there, so we hardly ever traverse much.
             */
            return findNodeFromTail(node);
        }
    
        /**
         * Returns true if node is on sync queue by searching backwards from tail.
         * Called only when needed by isOnSyncQueue.
         * @return true if present
         */
        private boolean findNodeFromTail(Node node) {
            Node t = tail;
            for (;;) {
                if (t == node)
                    return true;
                if (t == null)
                    return false;
                t = t.prev;
            }
        }
    
        /**
         * Transfers a node from a condition queue onto sync queue.
         * Returns true if successful.
         * @param node the node
         * @return true if successfully transferred (else the node was
         * cancelled before signal).
         */
        final boolean transferForSignal(Node node) {
            /*
             * If cannot change waitStatus, the node has been cancelled.
             */
            if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
                return false;
    
            /*
             * Splice onto queue and try to set waitStatus of predecessor to
             * indicate that thread is (probably) waiting. If cancelled or
             * attempt to set waitStatus fails, wake up to resync (in which
             * case the waitStatus can be transiently and harmlessly wrong).
             */
            Node p = enq(node);
            int ws = p.waitStatus;
            if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
                LockSupport.unpark(node.thread);
            return true;
        }
    
        /**
         * Transfers node, if necessary, to sync queue after a cancelled
         * wait. Returns true if thread was cancelled before being
         * signalled.
         * @param current the waiting thread
         * @param node its node
         * @return true if cancelled before the node was signalled
         */
        final boolean transferAfterCancelledWait(Node node) {
            if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
                enq(node);
                return true;
            }
            /*
             * If we lost out to a signal(), then we can't proceed
             * until it finishes its enq().  Cancelling during an
             * incomplete transfer is both rare and transient, so just
             * spin.
             */
            while (!isOnSyncQueue(node))
                Thread.yield();
            return false;
        }
    
        /**
         * Invokes release with current state value; returns saved state.
         * Cancels node and throws exception on failure.
         * @param node the condition node for this wait
         * @return previous sync state
         */
        final int fullyRelease(Node node) {
            boolean failed = true;
            try {
                int savedState = getState();
                if (release(savedState)) {
                    failed = false;
                    return savedState;
                } else {
                    throw new IllegalMonitorStateException();
                }
            } finally {
                if (failed)
                    node.waitStatus = Node.CANCELLED;
            }
        }
    
        // Instrumentation methods for conditions
    
        /**
         * Queries whether the given ConditionObject
         * uses this synchronizer as its lock.
         *
         * @param condition the condition
         * @return <tt>true</tt> if owned
         * @throws NullPointerException if the condition is null
         */
        public final boolean owns(ConditionObject condition) {
            if (condition == null)
                throw new NullPointerException();
            return condition.isOwnedBy(this);
        }
    
        /**
         * Queries whether any threads are waiting on the given condition
         * associated with this synchronizer. Note that because timeouts
         * and interrupts may occur at any time, a <tt>true</tt> return
         * does not guarantee that a future <tt>signal</tt> will awaken
         * any threads.  This method is designed primarily for use in
         * monitoring of the system state.
         *
         * @param condition the condition
         * @return <tt>true</tt> if there are any waiting threads
         * @throws IllegalMonitorStateException if exclusive synchronization
         *         is not held
         * @throws IllegalArgumentException if the given condition is
         *         not associated with this synchronizer
         * @throws NullPointerException if the condition is null
         */
        public final boolean hasWaiters(ConditionObject condition) {
            if (!owns(condition))
                throw new IllegalArgumentException("Not owner");
            return condition.hasWaiters();
        }
    
        /**
         * Returns an estimate of the number of threads waiting on the
         * given condition associated with this synchronizer. Note that
         * because timeouts and interrupts may occur at any time, the
         * estimate serves only as an upper bound on the actual number of
         * waiters.  This method is designed for use in monitoring of the
         * system state, not for synchronization control.
         *
         * @param condition the condition
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if exclusive synchronization
         *         is not held
         * @throws IllegalArgumentException if the given condition is
         *         not associated with this synchronizer
         * @throws NullPointerException if the condition is null
         */
        public final int getWaitQueueLength(ConditionObject condition) {
            if (!owns(condition))
                throw new IllegalArgumentException("Not owner");
            return condition.getWaitQueueLength();
        }
    
        /**
         * Returns a collection containing those threads that may be
         * waiting on the given condition associated with this
         * synchronizer.  Because the actual set of threads may change
         * dynamically while constructing this result, the returned
         * collection is only a best-effort estimate. The elements of the
         * returned collection are in no particular order.
         *
         * @param condition the condition
         * @return the collection of threads
         * @throws IllegalMonitorStateException if exclusive synchronization
         *         is not held
         * @throws IllegalArgumentException if the given condition is
         *         not associated with this synchronizer
         * @throws NullPointerException if the condition is null
         */
        public final Collection<Thread> getWaitingThreads(ConditionObject condition) {
            if (!owns(condition))
                throw new IllegalArgumentException("Not owner");
            return condition.getWaitingThreads();
        }
    
        /**
         * Condition implementation for a {@link
         * AbstractQueuedSynchronizer} serving as the basis of a {@link
         * Lock} implementation.
         *
         * <p>Method documentation for this class describes mechanics,
         * not behavioral specifications from the point of view of Lock
         * and Condition users. Exported versions of this class will in
         * general need to be accompanied by documentation describing
         * condition semantics that rely on those of the associated
         * <tt>AbstractQueuedSynchronizer</tt>.
         *
         * <p>This class is Serializable, but all fields are transient,
         * so deserialized conditions have no waiters.
         */
        public class ConditionObject implements Condition, java.io.Serializable {
            private static final long serialVersionUID = 1173984872572414699L;
            /** First node of condition queue. */
            private transient Node firstWaiter;
            /** Last node of condition queue. */
            private transient Node lastWaiter;
    
            /**
             * Creates a new <tt>ConditionObject</tt> instance.
             */
            public ConditionObject() { }
    
            // Internal methods
    
            /**
             * Adds a new waiter to wait queue.
             * @return its new wait node
             */
            private Node addConditionWaiter() {
                Node t = lastWaiter;
                // If lastWaiter is cancelled, clean out.
                if (t != null && t.waitStatus != Node.CONDITION) {
                    unlinkCancelledWaiters();
                    t = lastWaiter;
                }
                Node node = new Node(Thread.currentThread(), Node.CONDITION);
                if (t == null)
                    firstWaiter = node;
                else
                    t.nextWaiter = node;
                lastWaiter = node;
                return node;
            }
    
            /**
             * Removes and transfers nodes until hit non-cancelled one or
             * null. Split out from signal in part to encourage compilers
             * to inline the case of no waiters.
             * @param first (non-null) the first node on condition queue
             */
            private void doSignal(Node first) {
                do {
                    if ( (firstWaiter = first.nextWaiter) == null)
                        lastWaiter = null;
                    first.nextWaiter = null;
                } while (!transferForSignal(first) &&
                         (first = firstWaiter) != null);
            }
    
            /**
             * Removes and transfers all nodes.
             * @param first (non-null) the first node on condition queue
             */
            private void doSignalAll(Node first) {
                lastWaiter = firstWaiter = null;
                do {
                    Node next = first.nextWaiter;
                    first.nextWaiter = null;
                    transferForSignal(first);
                    first = next;
                } while (first != null);
            }
    
            /**
             * Unlinks cancelled waiter nodes from condition queue.
             * Called only while holding lock. This is called when
             * cancellation occurred during condition wait, and upon
             * insertion of a new waiter when lastWaiter is seen to have
             * been cancelled. This method is needed to avoid garbage
             * retention in the absence of signals. So even though it may
             * require a full traversal, it comes into play only when
             * timeouts or cancellations occur in the absence of
             * signals. It traverses all nodes rather than stopping at a
             * particular target to unlink all pointers to garbage nodes
             * without requiring many re-traversals during cancellation
             * storms.
             */
            private void unlinkCancelledWaiters() {
                Node t = firstWaiter;
                Node trail = null;
                while (t != null) {
                    Node next = t.nextWaiter;
                    if (t.waitStatus != Node.CONDITION) {
                        t.nextWaiter = null;
                        if (trail == null)
                            firstWaiter = next;
                        else
                            trail.nextWaiter = next;
                        if (next == null)
                            lastWaiter = trail;
                    }
                    else
                        trail = t;
                    t = next;
                }
            }
    
            // public methods
    
            /**
             * Moves the longest-waiting thread, if one exists, from the
             * wait queue for this condition to the wait queue for the
             * owning lock.
             *
             * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
             *         returns {@code false}
             */
            public final void signal() {
                if (!isHeldExclusively())
                    throw new IllegalMonitorStateException();
                Node first = firstWaiter;
                if (first != null)
                    doSignal(first);
            }
    
            /**
             * Moves all threads from the wait queue for this condition to
             * the wait queue for the owning lock.
             *
             * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
             *         returns {@code false}
             */
            public final void signalAll() {
                if (!isHeldExclusively())
                    throw new IllegalMonitorStateException();
                Node first = firstWaiter;
                if (first != null)
                    doSignalAll(first);
            }
    
            /**
             * Implements uninterruptible condition wait.
             * <ol>
             * <li> Save lock state returned by {@link #getState}.
             * <li> Invoke {@link #release} with
             *      saved state as argument, throwing
             *      IllegalMonitorStateException if it fails.
             * <li> Block until signalled.
             * <li> Reacquire by invoking specialized version of
             *      {@link #acquire} with saved state as argument.
             * </ol>
             */
            public final void awaitUninterruptibly() {
                Node node = addConditionWaiter();
                int savedState = fullyRelease(node);
                boolean interrupted = false;
                while (!isOnSyncQueue(node)) {
                    LockSupport.park(this);
                    if (Thread.interrupted())
                        interrupted = true;
                }
                if (acquireQueued(node, savedState) || interrupted)
                    selfInterrupt();
            }
    
            /*
             * For interruptible waits, we need to track whether to throw
             * InterruptedException, if interrupted while blocked on
             * condition, versus reinterrupt current thread, if
             * interrupted while blocked waiting to re-acquire.
             */
    
            /** Mode meaning to reinterrupt on exit from wait */
            private static final int REINTERRUPT =  1;
            /** Mode meaning to throw InterruptedException on exit from wait */
            private static final int THROW_IE    = -1;
    
            /**
             * Checks for interrupt, returning THROW_IE if interrupted
             * before signalled, REINTERRUPT if after signalled, or
             * 0 if not interrupted.
             */
            private int checkInterruptWhileWaiting(Node node) {
                return Thread.interrupted() ?
                    (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                    0;
            }
    
            /**
             * Throws InterruptedException, reinterrupts current thread, or
             * does nothing, depending on mode.
             */
            private void reportInterruptAfterWait(int interruptMode)
                throws InterruptedException {
                if (interruptMode == THROW_IE)
                    throw new InterruptedException();
                else if (interruptMode == REINTERRUPT)
                    selfInterrupt();
            }
    
            /**
             * Implements interruptible condition wait.
             * <ol>
             * <li> If current thread is interrupted, throw InterruptedException.
             * <li> Save lock state returned by {@link #getState}.
             * <li> Invoke {@link #release} with
             *      saved state as argument, throwing
             *      IllegalMonitorStateException if it fails.
             * <li> Block until signalled or interrupted.
             * <li> Reacquire by invoking specialized version of
             *      {@link #acquire} with saved state as argument.
             * <li> If interrupted while blocked in step 4, throw InterruptedException.
             * </ol>
             */
            public final void await() throws InterruptedException {
                if (Thread.interrupted())
                    throw new InterruptedException();
                Node node = addConditionWaiter();
                int savedState = fullyRelease(node);
                int interruptMode = 0;
                while (!isOnSyncQueue(node)) {
                    LockSupport.park(this);
                    if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                        break;
                }
                if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                    interruptMode = REINTERRUPT;
                if (node.nextWaiter != null) // clean up if cancelled
                    unlinkCancelledWaiters();
                if (interruptMode != 0)
                    reportInterruptAfterWait(interruptMode);
            }
    
            /**
             * Implements timed condition wait.
             * <ol>
             * <li> If current thread is interrupted, throw InterruptedException.
             * <li> Save lock state returned by {@link #getState}.
             * <li> Invoke {@link #release} with
             *      saved state as argument, throwing
             *      IllegalMonitorStateException if it fails.
             * <li> Block until signalled, interrupted, or timed out.
             * <li> Reacquire by invoking specialized version of
             *      {@link #acquire} with saved state as argument.
             * <li> If interrupted while blocked in step 4, throw InterruptedException.
             * </ol>
             */
            public final long awaitNanos(long nanosTimeout)
                    throws InterruptedException {
                if (Thread.interrupted())
                    throw new InterruptedException();
                Node node = addConditionWaiter();
                int savedState = fullyRelease(node);
                long lastTime = System.nanoTime();
                int interruptMode = 0;
                while (!isOnSyncQueue(node)) {
                    if (nanosTimeout <= 0L) {
                        transferAfterCancelledWait(node);
                        break;
                    }
                    LockSupport.parkNanos(this, nanosTimeout);
                    if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                        break;
    
                    long now = System.nanoTime();
                    nanosTimeout -= now - lastTime;
                    lastTime = now;
                }
                if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                    interruptMode = REINTERRUPT;
                if (node.nextWaiter != null)
                    unlinkCancelledWaiters();
                if (interruptMode != 0)
                    reportInterruptAfterWait(interruptMode);
                return nanosTimeout - (System.nanoTime() - lastTime);
            }
    
            /**
             * Implements absolute timed condition wait.
             * <ol>
             * <li> If current thread is interrupted, throw InterruptedException.
             * <li> Save lock state returned by {@link #getState}.
             * <li> Invoke {@link #release} with
             *      saved state as argument, throwing
             *      IllegalMonitorStateException if it fails.
             * <li> Block until signalled, interrupted, or timed out.
             * <li> Reacquire by invoking specialized version of
             *      {@link #acquire} with saved state as argument.
             * <li> If interrupted while blocked in step 4, throw InterruptedException.
             * <li> If timed out while blocked in step 4, return false, else true.
             * </ol>
             */
            public final boolean awaitUntil(Date deadline)
                    throws InterruptedException {
                if (deadline == null)
                    throw new NullPointerException();
                long abstime = deadline.getTime();
                if (Thread.interrupted())
                    throw new InterruptedException();
                Node node = addConditionWaiter();
                int savedState = fullyRelease(node);
                boolean timedout = false;
                int interruptMode = 0;
                while (!isOnSyncQueue(node)) {
                    if (System.currentTimeMillis() > abstime) {
                        timedout = transferAfterCancelledWait(node);
                        break;
                    }
                    LockSupport.parkUntil(this, abstime);
                    if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                        break;
                }
                if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                    interruptMode = REINTERRUPT;
                if (node.nextWaiter != null)
                    unlinkCancelledWaiters();
                if (interruptMode != 0)
                    reportInterruptAfterWait(interruptMode);
                return !timedout;
            }
    
            /**
             * Implements timed condition wait.
             * <ol>
             * <li> If current thread is interrupted, throw InterruptedException.
             * <li> Save lock state returned by {@link #getState}.
             * <li> Invoke {@link #release} with
             *      saved state as argument, throwing
             *      IllegalMonitorStateException if it fails.
             * <li> Block until signalled, interrupted, or timed out.
             * <li> Reacquire by invoking specialized version of
             *      {@link #acquire} with saved state as argument.
             * <li> If interrupted while blocked in step 4, throw InterruptedException.
             * <li> If timed out while blocked in step 4, return false, else true.
             * </ol>
             */
            public final boolean await(long time, TimeUnit unit)
                    throws InterruptedException {
                if (unit == null)
                    throw new NullPointerException();
                long nanosTimeout = unit.toNanos(time);
                if (Thread.interrupted())
                    throw new InterruptedException();
                Node node = addConditionWaiter();
                int savedState = fullyRelease(node);
                long lastTime = System.nanoTime();
                boolean timedout = false;
                int interruptMode = 0;
                while (!isOnSyncQueue(node)) {
                    if (nanosTimeout <= 0L) {
                        timedout = transferAfterCancelledWait(node);
                        break;
                    }
                    if (nanosTimeout >= spinForTimeoutThreshold)
                        LockSupport.parkNanos(this, nanosTimeout);
                    if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                        break;
                    long now = System.nanoTime();
                    nanosTimeout -= now - lastTime;
                    lastTime = now;
                }
                if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                    interruptMode = REINTERRUPT;
                if (node.nextWaiter != null)
                    unlinkCancelledWaiters();
                if (interruptMode != 0)
                    reportInterruptAfterWait(interruptMode);
                return !timedout;
            }
    
            //  support for instrumentation
    
            /**
             * Returns true if this condition was created by the given
             * synchronization object.
             *
             * @return {@code true} if owned
             */
            final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
                return sync == AbstractQueuedSynchronizer.this;
            }
    
            /**
             * Queries whether any threads are waiting on this condition.
             * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
             *
             * @return {@code true} if there are any waiting threads
             * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
             *         returns {@code false}
             */
            protected final boolean hasWaiters() {
                if (!isHeldExclusively())
                    throw new IllegalMonitorStateException();
                for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                    if (w.waitStatus == Node.CONDITION)
                        return true;
                }
                return false;
            }
    
            /**
             * Returns an estimate of the number of threads waiting on
             * this condition.
             * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
             *
             * @return the estimated number of waiting threads
             * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
             *         returns {@code false}
             */
            protected final int getWaitQueueLength() {
                if (!isHeldExclusively())
                    throw new IllegalMonitorStateException();
                int n = 0;
                for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                    if (w.waitStatus == Node.CONDITION)
                        ++n;
                }
                return n;
            }
    
            /**
             * Returns a collection containing those threads that may be
             * waiting on this Condition.
             * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
             *
             * @return the collection of threads
             * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
             *         returns {@code false}
             */
            protected final Collection<Thread> getWaitingThreads() {
                if (!isHeldExclusively())
                    throw new IllegalMonitorStateException();
                ArrayList<Thread> list = new ArrayList<Thread>();
                for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                    if (w.waitStatus == Node.CONDITION) {
                        Thread t = w.thread;
                        if (t != null)
                            list.add(t);
                    }
                }
                return list;
            }
        }
    
        /**
         * Setup to support compareAndSet. We need to natively implement
         * this here: For the sake of permitting future enhancements, we
         * cannot explicitly subclass AtomicInteger, which would be
         * efficient and useful otherwise. So, as the lesser of evils, we
         * natively implement using hotspot intrinsics API. And while we
         * are at it, we do the same for other CASable fields (which could
         * otherwise be done with atomic field updaters).
         */
        private static final Unsafe unsafe = Unsafe.getUnsafe();
        private static final long stateOffset;
        private static final long headOffset;
        private static final long tailOffset;
        private static final long waitStatusOffset;
        private static final long nextOffset;
    
        static {
            try {
                stateOffset = unsafe.objectFieldOffset
                    (AbstractQueuedSynchronizer.class.getDeclaredField("state"));
                headOffset = unsafe.objectFieldOffset
                    (AbstractQueuedSynchronizer.class.getDeclaredField("head"));
                tailOffset = unsafe.objectFieldOffset
                    (AbstractQueuedSynchronizer.class.getDeclaredField("tail"));
                waitStatusOffset = unsafe.objectFieldOffset
                    (Node.class.getDeclaredField("waitStatus"));
                nextOffset = unsafe.objectFieldOffset
                    (Node.class.getDeclaredField("next"));
    
            } catch (Exception ex) { throw new Error(ex); }
        }
    
        /**
         * CAS head field. Used only by enq.
         */
        private final boolean compareAndSetHead(Node update) {
            return unsafe.compareAndSwapObject(this, headOffset, null, update);
        }
    
        /**
         * CAS tail field. Used only by enq.
         */
        private final boolean compareAndSetTail(Node expect, Node update) {
            return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
        }
    
        /**
         * CAS waitStatus field of a node.
         */
        private static final boolean compareAndSetWaitStatus(Node node,
                                                             int expect,
                                                             int update) {
            return unsafe.compareAndSwapInt(node, waitStatusOffset,
                                            expect, update);
        }
    
        /**
         * CAS next field of a node.
         */
        private static final boolean compareAndSetNext(Node node,
                                                       Node expect,
                                                       Node update) {
            return unsafe.compareAndSwapObject(node, nextOffset, expect, update);
        }
    }
    View Code

    0. 前言

    AbstractQueuedSynchronizer,简称AQS,Doug Lea大神的作品,jsr166导入,可以说是J.U.C包的核心,有好几个重要的同步工具是基于AQS实现的。

    代码量2330行,较多,且相对难以理解,我也不敢保证能用这一篇博文就将它讲清楚,只能尽力而为吧。

    本文主要是参考Doug Lea的 <The java.util.concurrent Synchronizer Framework>译文地址

    以及并发编程网上的系列文章写成

    1. CLH锁(Craig, Landin, and Hagersten locks)

    参见我写的另外一篇博客《Ticket Lock, CLH Lock, MCS Lock

    AQS使用的CLH锁,而不是更加优化的MCS锁,Doug Lea对此的解释是:"However, they appeared more amenable than MCS for use in the synchronzier framework because they are more easily adapted to handle cancellation and timeouts, so were chosen as a basis." 大概意思是说,CLH锁比MCS锁更容易实现取消与超时的操作。(可能是说在CLH锁中,如果想要取消某个等待线程,只需要直接更新对应节点的状态位即可。如果是MCS锁则需要更新后继节点的状态位,较为麻烦)

    需要注意的是,AQS使用的不是原版CLH锁,而是在CLH锁的基础上做了两个改动

    a. 原版的CLH锁是单链表,每个节点维护了一个指向前驱节点的指针。AQS使用的版本是双链表,每个节点都维护了指向前驱节点与后继节点的指针

      分析:原版的CLH锁认为所有的未获得锁的线程都在自旋等待,所以如果前驱节点的状态位发生了改变,后继节点对应的线程可以第一时间感知到。但在实际应用中,自旋是非常消耗系统资源的(一个空转自旋的线程就能占满一个core),所以等待线程一般在获取不到锁之后就会自动等待(调用park进入WAITING状态),所以前驱节点在释放锁之后,必须还要将后继节点唤醒(unpark)。这样就需要多维护一个指向后继节点的向后指针。

    但是由于没有办法维护向双向链表中插入节点操作的原子性,所以如果看到一个节点的后继指针为null,不能简单的认为真的已经没有后继节点了(可能后继节点已经将tail更新,但是还没来得及维护它的前驱节点的向后指针,也就是虽然节点已经插入链表,但是从链表头不能遍历到这个节点),而是要做一些微妙的判断,后文会提及(unparkSuccessor方法)。

    b. AQS版本的CLH锁中的状态位用于控制阻塞而非自旋

      分析: 原版CLH锁只要看到状态位被标记为unlock就能停止自旋并返回了。而AQS版本的CLH锁中,是否停止自旋跟前驱节点的状态位没什么关系,只有在调用tryAcquire方法成功时,才能停止自旋并返回。节点的状态位主要是为了让当前线程在调用tryAcquire方法失败后,是否将自身park住而存在的(参见acquireQueued函数

    2. 等待队列中节点的结构

    static final class Node {
            /** Marker to indicate a node is waiting in shared mode */
            static final Node SHARED = new Node();//addWaiter时标记新添加的节点为共享模式
            /** Marker to indicate a node is waiting in exclusive mode */
            static final Node EXCLUSIVE = null;//addWaiter时标记新添加的节点为独占模式
    
            /** waitStatus value to indicate thread has cancelled */
            static final int CANCELLED =  1;//此节点对应的线程已被取消或者超时,跳过
            /** waitStatus value to indicate successor's thread needs unparking */
            static final int SIGNAL    = -1;//此节点的后继节点需要被唤醒
            /** waitStatus value to indicate thread is waiting on condition */
            static final int CONDITION = -2;//当前节点正在等待条件
            /**
             * waitStatus value to indicate the next acquireShared should
             * unconditionally propagate
             */
            static final int PROPAGATE = -3;//后续的acquireShared可以执行
    
            volatile int waitStatus;//节点状态,与上面的几条对应
    
            volatile Node prev;//前驱节点
    
            volatile Node next;//后继节点
    
            volatile Thread thread;//当前节点对应的线程
    
            Node nextWaiter;//条件队列中的后继节点
    }

    3. AQS的几个关键属性

        /**
         * Head of the wait queue, lazily initialized.  Except for
         * initialization, it is modified only via method setHead.  Note:
         * If head exists, its waitStatus is guaranteed not to be
         * CANCELLED.
         */
        private transient volatile Node head;//CLH队列的头结点
    
        /**
         * Tail of the wait queue, lazily initialized.  Modified only via
         * method enq to add new wait node.
         */
        private transient volatile Node tail;//CLH队列的尾结点
    
        /**
         * The synchronization state.
         */
        private volatile int state;//AQS的同步状态变量,例如在ReentrantLock中,state == 0表示无锁,state > 0表示有锁, state不为0时,其值表示锁重入的次数。设置为volatile是因为它可能被多线程修改。

    4. 示例

    为了便于理解,我先写一个简单的排它锁的例子(只能被一个线程持有,不考虑重入),后续分析会基于这个例子进行

    public class Mutex implements Lock {
        //自定义的同步器,继承于AQS
        private static class Sync extends AbstractQueuedSynchronizer {
            //state==0 : 无锁,state == 1 : 有锁
            @Override
            protected boolean isHeldExclusively() {
                return getState() == 1;
            }
    
            @Override
            public boolean tryAcquire(int acuires) {
                //尝试把state从0更新成1,成功的话说明占锁成功,失败则反之
                if (compareAndSetState(0, 1)) {
                    setExclusiveOwnerThread(Thread.currentThread());
                    return true;
                }
                return false;
            }
    
            //释放锁
            @Override
            protected boolean tryRelease(int releases) {
                //如果当前线程与锁的持有线程不等,说明有unlock与lock的线程不成对,则抛出异常
                if (Thread.currentThread() != getExclusiveOwnerThread()) {
                    throw new IllegalMonitorStateException();
                }
                setExclusiveOwnerThread(null);
                setState(0);
                return true;
            }
    
            Condition newCondition() {
                return new ConditionObject();
            }
        }
    
        private final Sync sync = new Sync();
    
        @Override
        public void lock() {
            sync.acquire(1);
        }
    
        @Override
        public void lockInterruptibly() throws InterruptedException {
            sync.acquireInterruptibly(1);
        }
    
        @Override
        public boolean tryLock() {
            return sync.tryAcquire(1);
        }
    
        @Override
        public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
            return sync.tryAcquireNanos(1, unit.toNanos(time));
        }
    
        @Override
        public void unlock() {
            sync.release(1);
        }
    
        @Override
        public Condition newCondition() {
            return sync.newCondition();
        }

    可以看到Mutex从Lock继承来的几个方法,都是通过调用Sync来实现的

    5. Mutex.lock的调用轨迹

    Mutex.lock会调用AQS的acquire方法,其源码如下

    public final void acquire(int arg) {
         if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

    acquire方法会先尝试调用被Mutex的Sync内部类重写的tryAcquire方法,此方法会尝试将state变量cas的从0修改为1。

    cas成功,说明Mutex已经被本线程独占,需要进一步修改AQS的线程标志,tryAcquire返回true

    cas失败,说明state不为0,说明Mutex已经被其他线程占据,返回false,然后走后续的流程,也就是执行:acquireQueued(addWaiter(Node.EXCLUSIVE), arg)

    先从内层的addWaiter函数看起

        /**
         * Creates and enqueues node for current thread and given mode.
         *
         * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
         * @return the new node
         */
        private Node addWaiter(Node mode) {
            Node node = new Node(Thread.currentThread(), mode);//创建等待队列节点
            // Try the fast path of enq; backup to full enq on failure//fast path是指无竞争的情况
            Node pred = tail;//取等待队列的尾结点,尝试向等待队列的队尾添加元素
            if (pred != null) {
                node.prev = pred;
                if (compareAndSetTail(pred, node)) {//尝试将tail更新为当前节点
                    pred.next = node;//更新成功,将当前节点的prev指针更新为之前的tail节点
                    return node;
                }
            }
            enq(node);//tail为空(等待队列没有初始化),或者cas更新tail失败,调用enq函数
            return node;
        }

    大意就是:新建节点,并且试图将其插入到等待队列的末尾,在无竞争的情况下插入操作直接成功,有竞争时插入可能失败,此时需要调用enq方法继续处理

        /**
         * Inserts node into queue, initializing if necessary. See picture above.
         * @param node the node to insert
         * @return node's predecessor
         */
        private Node enq(final Node node) {
            //由于竞争,插入节点可能失败,所以需要无限循环,直到节点插入等待队列成功为止
            for (;;) {
                Node t = tail;
                //AQS采用了延迟加载的策略,也就是等待队列一开始是没有元素的,head==tail==null,所以如果检测到tail==null,需要新建一个dummy节点并插入等待队列中
           //当然这个插入操作也必须是原子的,在有其他线程竞争时有失败的可能,失败则重试
    if (t == null) { // Must initialize if (compareAndSetHead(new Node())) tail = head; } else { //跟addWaiter中一样的操作,尝试向等待队列的尾部插入元素,如果插入失败则重试 node.prev = t; if (compareAndSetTail(t, node)) { t.next = node; return t; } } } }

    概括一下,addWaiter(Node.EXCLUSIVE),这句代码的意思就是新建一个节点并插入到等待队列的尾部,这个插入过程是lock-free的,返回结果是新建的那个节点

    现在我们来看acquireQueued函数

        /**
         * Acquires in exclusive uninterruptible mode for thread already in
         * queue. Used by condition wait methods as well as acquire.
         *
         * @param node the node
         * @param arg the acquire argument
         * @return {@code true} if interrupted while waiting
         */
        final boolean acquireQueued(final Node node, int arg) {
            boolean failed = true;
            try {
                boolean interrupted = false;
                for (;;) {//无限循环,因为线程有可能被错误的唤醒或者处于自旋状态
                    final Node p = node.predecessor();//获取node的前驱节点
                    if (p == head && tryAcquire(arg)) {//如果前驱节点是队头->此线程前面只有一个->tryAcquire成功->占领临界区成功
                        setHead(node);//将自己设为队头
                        p.next = null; // help GC
                        failed = false;
                        return interrupted;
                    }
                    //acquire失败后根据前驱节点的状态选择是否将本线程park住
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())//此方法会把本线程park住,如果从park状态退出(可能是unpark,也可能是被中断),会检查线程的interrupt位并返回
                        interrupted = true;//如果线程是因为中断而返回的,标记一下
                }
            } finally {
                if (failed)//抛出异常时,failed才会为true
                    cancelAcquire(node);//将本节点的状态标记为cancel,后续处理的时候会被跳过
            }
        }

    大概意思就是让这个线程自旋,每自旋一次就检查一下前驱节点是否为队头,如果是的话就tryAcquire一下,tryAcquire成功了就说明获取临界区成功了,函数可以返回。如果失败就把自己park住。

    如果线程从park状态中解除,可能是两种情况

    a. 前面的线程把本线程unpark了,那赶紧自旋看看能不能tryAcquire成功,不能就继续park

    b. 本线程被中断了,parkAndCheckInterrupt函数返回true,那么标记interrupted为true,然后还是继续自旋看看能不能tryAcquire成功,不能就继续park(acquireQueued方法是不响应中断的,想要对中断做出反应的话需要使用doAcquireInterruptibly方法)

    再分析一下中间使用到的shouldParkAfterFailedAcquire方法

        /**
         * Checks and updates status for a node that failed to acquire.
         * Returns true if thread should block. This is the main signal
         * control in all acquire loops.  Requires that pred == node.prev
         *
         * @param pred node's predecessor holding status
         * @param node the node
         * @return {@code true} if thread should block
         */
        private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
            int ws = pred.waitStatus;//获取前驱节点的状态
            if (ws == Node.SIGNAL)//前驱节点为SIGNAL,说明本线程还在等待其他线程唤醒,需要继续park
                /*
                 * This node has already set status asking a release
                 * to signal it, so it can safely park.
                 */
                return true;
            if (ws > 0) {//只有CANCELLED这一种情况,此时需要跳过这个前驱节点直到找到一个状态位不为CANCELLED的节点
                /*
                 * Predecessor was cancelled. Skip over predecessors and
                 * indicate retry.
                 */
                //循环跳过节点
                do {
                    node.prev = pred = pred.prev;
                } while (pred.waitStatus > 0);
                pred.next = node;
            } else {
                /*
                 * waitStatus must be 0 or PROPAGATE.  Indicate that we
                 * need a signal, but don't park yet.  Caller will need to
                 * retry to make sure it cannot acquire before parking.
                 */
           //前驱节点的状态不是SIGNAL,那么将其强制修改为SIGNAL,这样才能放心的将自己park,否则可能会丢失消息
    compareAndSetWaitStatus(pred, ws, Node.SIGNAL); } return false; }

     acquire方法的最后一行意思是说:如果acquire的过程中这个线程被中断过,它就会调用Thread.currentThread().interrupt()方法自我中断一下,除了设置线程的中断标记位以外并无卵用。

    总结一下,如果Mutex被其他线程占用,当前线程调用Mutex.lock后,会在Mutex的AQS的等待队列中创建一个节点并自我park,等待前面的工作线程将其唤醒。如果Mutex.lock方法返回了,说明当前线程已经拿到了Mutex的独占锁,可以放心使用临界区的资源了。

    6. Mutex.unlock的调用轨迹

    Mutex.unlock会调用AQS的release方法,其源码如下

        /**
         * Releases in exclusive mode.  Implemented by unblocking one or
         * more threads if {@link #tryRelease} returns true.
         * This method can be used to implement method {@link Lock#unlock}.
         *
         * @param arg the release argument.  This value is conveyed to
         *        {@link #tryRelease} but is otherwise uninterpreted and
         *        can represent anything you like.
         * @return the value returned from {@link #tryRelease}
         */
        public final boolean release(int arg) {
            if (tryRelease(arg)) {//尝试释放锁
                Node h = head;//获取等待队列的头结点
                if (h != null && h.waitStatus != 0)
                    unparkSuccessor(h);//唤醒等待队列中的后继线程
                return true;
            }
            return false;
        }

    tryRelease方法是Mutex中的Sync重写的

    先检测AQS维护的拥有者线程,如果与当前线程不匹配,说明这个线程正在释放不属于自己的Mutex,抛出异常。

    然后将拥有者线程设置为null

    最后将AQS的状态为设置为0,让出Mutex,标记Mutex处于无锁状态

    如果tryRelease成功,再去检查AQS的等待队列中是否还有线程正在等待,如果有,将其唤醒。

    unparkSuccessor方法源码如下

        /**
         * Wakes up node's successor, if one exists.
         *
         * @param node the node
         */
        private void unparkSuccessor(Node node) {//传入的是等待队列的头结点
            /*
             * If status is negative (i.e., possibly needing signal) try
             * to clear in anticipation of signalling.  It is OK if this
             * fails or if status is changed by waiting thread.
             */
            int ws = node.waitStatus;
            if (ws < 0)
                compareAndSetWaitStatus(node, ws, 0);
    
            /*
             * Thread to unpark is held in successor, which is normally
             * just the next node.  But if cancelled or apparently null,
             * traverse backwards from tail to find the actual
             * non-cancelled successor.
             */
            Node s = node.next;//一般来说头结点的后继节点就是等待节点了,但是有时等待节点可能已经被cancel或者一时找不到后继节点(节点已经被插入到队列尾部,但是还没来得及维护前驱节点的向后指针),那就需要从等待队列的尾部回溯,直到找到离队头最近的等待被唤醒的节点为止
            if (s == null || s.waitStatus > 0) {
                s = null;
                for (Node t = tail; t != null && t != node; t = t.prev)
                    if (t.waitStatus <= 0)
                        s = t;
            }
            if (s != null)//如果找到等待被唤醒的节点,unpark对应的线程
                LockSupport.unpark(s.thread);
        }

    总结一下,Mutex.unlock方法会解除工作线程对Mutex的占用,然后去AQS的等待队列里寻找,如果找到等待中的线程,则将其唤醒。

    7. 总结

    AQS中已经完成了解决线程竞争问题的大部分操作,如果想要实现自定义的线程同步工具,最好直接继承AQS

    上面讲解的只是AQS的一部分内容,如果全部介绍的话文章篇幅太长就没法看了(我感觉现在已经没法看了)

    其他内容留到分析J.U.C的其他工具类的时候再做讲解。

    水平有限,如有错误请留言指正

    参考资料

     <The java.util.concurrent Synchronizer Framework>译文地址

    Ticket Lock, CLH Lock, MCS Lock

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  • 原文地址:https://www.cnblogs.com/stevenczp/p/7136427.html
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